On your truck, is there a signal from the air conditioning that could be telling the fan to go into high mode ? On my Nissan, there is, and it has to do with when the A/C has higher head pressure ( hotter day, A/C working harder ). Just something else to consider. I have no specific knowledge of how the Ford works.
EDIT: my point is, that at least on the Nissan, you "could" have normal water temps, but yet still get maximum fan outputs due to a signal from the A/C system. I'm leaving out some of the Nissan details here because it has two fans with multiple possible op modes, but I hope you can see my point.
